(i) Field of the Invention
This invention relates to a replacement heart valve and particularly to a two leaflet replacement heart valve.
(ii) Description of the Prior Art
The human heart has four valves which when properly functioning allow unidirectional blood flow. Heart valve disease in advanced forms causes severe disability or death. The quality and length of life for patients suffering from valve disease can be remarkably improved by surgical treatment, which usually involves the total replacement of the diseased valve with a prosthetic valve. When natural valves malfunction they can be replaced, by a variety of prosthetic heart valves, in order to restore effective blood flow.
Since the first successful implantation in a human, nearly 50 different valve types have been introduced and many have been discarded; of those remaining, two basic types are in use--those with occluders constructed of human or animal tissues (tissue valves) and those with occluders constructed of various metals, carbon, and plastic components (nontissue or mechanical valves). These devices have come in various forms of flexible unicusp, bicuspid, and tricuspid valves, ball valves and "butterfly" or flapper valves. The mechanical valves have one or more rigid occluders, e.g. discs or balls which slide or tilt in a framework, and are generally made of titanium or hardened graphite. Tissue valves were developed in an attempt to eliminate some of the problems, e.g., noisy operation and a tendency to cause blood clotting, which arose with all of the mechanical valves. Flexible leaflets for tissue valves are usually made of chemically denatured biological tissues, e.g. whole porcine aortic valves and bovine pericardium. The mechanical valves require lifelong use of anticoagulants by the patient, and the long-term durability of the tissue valves is in question. It is believed that a mitral prosthesis which anatomically resembles the natural bileaflet valve is more likely to produce and take advantage of physiologic ventricular flow patterns which appear to be associated with efficient natural valve closure. However, the extent to which these flow patterns aid in the closure of the mitral valve is still undetermined. Also, from a surgical point of view, a mitral valve having only two struts projecting into the ventricle may in some cases be suitable for implantation where the use of a three leaflet valve would be difficult. At present, the only flexible occluder prostheses commercially available are those having three leaflets.
Significant late complications following implantation of these valves can occur and are related to valve design and materials. Present valve replacements do not permit restoration of normal pressure-flow dynamics at all levels of cardiac function. Thus, there is still no clear-cut choice for the surgeon of what valve to use and the search for the ideal replacement valve is continuing.
Stented tissue valves, that is, frame supported valvular grafts which may be either xenografts (heterografts) or allografts (homografts), have been used as replacement heart valves. (See, for example, Carpentier et al., J. Thorac. Cardiovasc, Surg. 68: 771 (1974); Zuhdi et al., Ann. Thorac. Surg. 17: 479 (1974); Horowitz et al., J. Thorac. Cardiovasc. Surg. 767: 885 (1974). In general, such grafts have been mounted on supporting frames (stents) which provide rigid orifice rings (see Weldon et al., J. Surg. Research 6: 548 (1966). Some stents have included struts capable of flexing inwardly to a limited extent, thereby reducing stresses imposed on the valve leaflets and decreasing possible erosion of surrounding cardiac tissues of the patient (see Sugie et al., J. Thorac. Cardiobasc. Surg. 57: 455 (1969); and Hardy, Human Organ Support and Replacement, 338 et. seq.). Despite the encouraging results with prosthetic tissue heart valves and in contrast to non-tissue prosthetic valves, there is a continuing need for improvement, particularly with regard to the hydrodynamic performance and long-range durability of the tissue valves.
The art is still faced with the desirability of providing an improved stent for a tissue (xenograft or allograft) heart valve which is capable of yielding to a limited extent in response to forces which tend to alter the configuration and circumference of the orifice ring, thereby improving the hydrodynamics and long term reliability of the valves. Accordingly, continued efforts are being made to develop more efficient, reliable and biocompatible prostheses.
Several investigators have studied the stresses to which natural and prosthetic valve leaflets are exposed in an attempt to improve the longterm structural integrity of leaflet valves. One investigator has provided a synthetic trileaflet aortic valve prosthesis. A seamless leaflet valve has been developed by another investigator and this valve has been provided with geometry designed to reduce shear stress at the commissures. There are few flexible leaflet prostheses made specifically for the mitral position. The search for improved devices continues because present valve designs can limit patient activity and can produce significant late complications.
The results of such continued efforts are evidenced in heart valves which are disclosed in issued United States patents.
U.S. Pat. No. 2,832,078 issued Apr. 29, 1958 to D. T. Williams discloses an aortic heart valve including a slotted cylindrical sheel with an internal three-sac membrane to provide opening and closing ports, which seal at the centre of the cylindrical shell.
U.S. Pat. No. 3,197,788 issued Aug. 3, 1965 to F. J. Segger provides an aortic heart valve including a deformable cone-shaped cusp-supporting ring, with the cusps having smooth curved surfaces.
U.S. Pat. No. 3,548,418 issued Dec. 22, 1970 to W. W. Angell et al. provides a graft-supporting ring for grafting porcine aortic valves in which the ring is generally in the form of the residual portion of a conical shell, having three struts, the ring being completely covered and having three internal depressed valve cusps.
U.S. Pat. No. 3,570,014 issued Mar. 16, 1971 to W. D. Hancock provides a stent for aortic and mitral heart valves in which the stent includes a ring and three support arms rising therefrom, to which commissures and cusps of a heart valve are attached.
U.S. Pat. No. 3,714,671 issued Feb. 6, 1973 to W. S. Edwards et al. provides a stent for supporting a tricuspid heart valve, in which the ring comprises portions of ellipses, in which the upstanding portions are covered with fabric and which terminate in radial wings, and to which three valve cusps are sutured, the valve cusps having straight trimmed edges, and being supported without tension.
U.S. Pat. No. 3,736,598 issued June 5, 1973 to B. J. Bellhouse et al. provides an aortic valve including a ring having three legs folded to U-shaped sections to which are attached three valve cusps whose free edges meet in radial planes of abutment.
U.S. Pat. No. 3,739,402 issued June 19, 1973 to D.A. Cooley et al. provides a graft support for a bicusp valve which includes a frusto-conical ring and a pair of inverted frusto-conical segments defining struts, all provided with a fabric cover, to which are secured a pair of cusps whose upper edges lie adjacent to each other to form the valve opening.
U.S. Pat. No. 3,733,062 issued July 10, 1973 to V. Parsonnet provides a heart valve construction including a stent having three lower arcuate portions and three upstanding posts, to which a fabric sheath is secured, and from which three valve leaflets, each having an arcuate edge and a straight edge are secured, so that the straight edges provide an upper meeting closure.
U.S. Pat. No. 3,755,823 issued Sept. 4, 1973 to W. D. Hancock provides a stent for heart valves in the form of a flexible stent including a ring having three spaced-apart apexes to which a cloth sleeve is attached and to which three valve cusps are attached , so that the free edges sag towards the centre, at which point they meet at a central, slightly raised point. This valve utilizes a whole porcine aortic valve which is pretreated before mounting on the stent.
U.S. Pat. No. 3,938,197 issued Feb. 17, 1976 to S. Milo provides a heart valve including a ring to which are attached a plurality of flat valve flaps whose free edges all meet in abutting relation.
U.S. Pat. No. 3,983,581 issued Oct. 5, 1976 to W. W. Angell et al. provides a heart valve stent of a particular shape, to which a covering is attached, and from which three valve cusps are attached so that their free edges meet at three commissures, and so that their common points meet at a central depression. A whole porcine xenograft is mounted to the stent.
U.S. Pat. No. 4,035,849 issued July 19, 1977 to W. W. Angell et al. provides a heart valve stent of a particular shape, to which a covering having a bead along its perimeter is attached and from which three valve cusps are attached, so that their free edges meet at three commissures and so that their common points meet at a central depression. A whole porcine xenograft is mounted to the stent.
U.S. Pat. No. 4,084,268 issued Apr. 18, 1978 to M. I. Ionescu et al. provides a heart valve including a dish-shaped cloth-covered stent having three upright posts, to which three cusps are attached, the cusps meeting at their upper edges at a flat closed portion, and in which the knots of the stitches are covered by a pledget and cover. This valve uses pretreated bovine pericardium for its three leaflets.
U.S. Pat. No. 4,106,129 issued Aug. 15, 1978 to A. F. Carpentier et al. provides a heart valve including a deformable wireframe stent having three inverted U-shaped commissure supports, to which are secured a cover, and from which are suspended three valve leaflets meeting along the commissures. A whole porcine xenograft is mounted to the stent.
U.S. Pat. No. 4,164,046 issued Aug. 14, 1979 to D. A. Cooley provides a mitral or tricuspid valve replacement which is based on an open ring stent.
U.S. Pat. No. 4,172,295 issued Oct. 30, 1979 to R. J. Batter provides a tricuspid heart valve dish-shaped cloth-covered stent having three upright ports to which are attached three cusps meeting at their upper edges at a flat closed position, in which the knots of the stitches are covered by a pledget and cover, and in which securing holes are provided between the cusps.
U.S. Pat. No. 4,178,639 issued Dec. 18, 1979 to J. C. Bokros provides a heart valve having an annular valve body and a pair of pivotally secured valve leaflets.
U.S. Pat. No. 3,739,402, to Cooley shows a stent which is a ring generally oblong in form and having a pair of projecting struts either extending perpendicular to the ring or outwardly inclined. The stent is covered with a fabric cover. The covered stent supports a pair of cusps defined by a tubular tissue covering around the struts and intervening space. The cusps, when the valve is in its closed position, follows a horizontal path between the struts.
U.S. Pat. No. 3,608,097 to Bellhouse et al discloses a tubular valve having at least three cusp-like pliable elements. The cusps are space 120.degree. apart and, when opened, form a cylindrical opening.
U.S. Pat. No. 4,222,126 to Boreless et al discloses a three leaflet heart valve with a semi-rigid frame of a bore ring and three struts and an integral elastomeric membrane which provides the three leaflets. The transition between the frame and the leaflet is tapered.
U.S. Pat. No. 4,275,469 patented June 30, 1981 by S. Gabbay provided a novel prosthetic heart valve. The valve included a tubular membrane having a flexible generally circular inlet end adapted to be attached to the annulus of a heart. One side of the tube was held to the heart cavity as by attachment to the papillary muscle. The other side of the tube was formed as an extended single flap adapted to move toward and away from the membrane on the attached side. This provided a closed or open valve at the outlet end.
Another valve structure was disclosed at the ESAO Proceedings at Brussels, Belgium, Sept. 1-3, 1982 in a paper by M. M. Black et al. That paper refers to Black et al United Kingdom Patent Application No. 8,201,793, which provides a bicuspid bioprosthetic mitral heart valve including a pair of leaflets secured to a valve base whose ring thickness varies to provide differential flexibility in the plane of the valve base, and to valve ports. The leaflet is cut from a flat sheet of fully fixed tissue originating from a conical solid having only one axis of curvature. The valve leaflet is derived from a conical surface that can buckle from one stable geometry to another so that when the two leaflets buckle inwards and their free edges coapt, a closed valve configuration obtains.
U.S. Pat. No. 4,340,977 of Richard T. Brownlee et al, provided a stented mitral heart valve which overcame many of the deficiencies of the prior art heart valves. This mitral heart valve had stent including a circular base and a pair of diametrically opposed struts, separating a pair of diametrically opposed, arcuately shaped, depressed reliefs, each such relief being bounded by a smooth curve interconnecting the struts to the circular base; a flexible, durable biocompatible covering secured to the stent and providing two equal, opposed, molded, flappably-movable, valve leaflets secured along the smooth curve defining the upper perimeter of the reliefs; the valve leaflets each being preformed and molded so that the free margins of the biocompatible covering along the free edge of each of the leaflets between the tips of each associated strut is so related to the circumference of the circular base, that when the valve is in its open position, the cross-sectional area of the exit is substantially equal to the cross-sectional area of the inside of the circular base, and, when the valve is in its relaxed and natural closed position, the shape of the leaflets is such that the free edges of the leaflets drop down and sealingly meet in substantially wrinkle-free form at a curve of apposition in the plane defined by the tips of the struts and the axis of the valve, and flow the approximate shape of a catenary curve.
Nevertheless even that mitral valve has not solved all the problems.